The present invention relates to a CRT display apparatus and more particularly to a wideband video output circuit suitable for a grid drive system.
Initially in the age of vacuum tube-type television receivers, video output circuits heretofore used have been of the type which differentially drives the first grid electrode G.sub.1 and the cathode electrode of a CRT.
This kind of conventional cathode-drive type display apparatus employing vacuum tubes has been disclosed in the magazine by D. G. Fink, "Television Engineering Handbook", McGraw-Hill Book Company, pp 16-255, 1957.
Also, the above kind of conventional griddrive type display apparatus employing vacuum tubes has likewise been disclosed on pages 16 to 257 of the same magazine.
Since display apparatus have subsequently been constructed with transistors, however, a cathode single-drive system has come into use. One of the reasons for cathode single-drive system coming into use has resided in that the electrostatic capacity of the first grid electrode G.sub.1 has a magnitude of about 1.6 times that of the cathode electrode K. As is well known in the art, the required power of the video output circuit is proportional to the load capacity. Therefore, the first grid G.sub.1 -drive system has not been used due to it requiring a large power consumption.
FIG. 1 is a diagram showing the technique of a conventional CRT display apparatus. The reason for the electrostatic capacity of the first grid G.sub.1 being so large relatively will now be described with reference to FIG. 1. In the Figure, numeral 1 designates a CRT, 2 a cathode electrode, 3 a first grid electrode G.sub.1, 4 a second grid electrode G.sub.2, 5 a cathode voltage application terminal, 6 a first grid voltage application terminal, and 7 a second grid voltage source which applies a dc voltage of about 700 V.
C.sub.KS is the cathode electrostatic capacity when the first grid G.sub.1 is grounded, and C.sub.IS the first grid electrostatic capacity when the cathode K is grounded. They are given by the following equations. EQU C.sub.KS =C.sub.K1+C.sub.KK .apprxeq.5.sub.p F (1) EQU C.sub.IS =C.sub.K1+C.sub.11 +C.sub.12 .apprxeq.8.sub.p F (2)
where
C.sub.K1 is the storage capacity of about 4.sub.p F between the cathode K and the first grid G.sub.1 ; PA1 C.sub.KK is the storage capacity of about 1.sub.p F between the cathode K and the ground; PA1 C.sub.11 is the storage capacity of about 1.sub.p F between the first grid G.sub.1 and the ground; and PA1 C.sub.12 is the storage capacity of about 3.sub.p F between the first grid G.sub.1 and the second grid G.sub.2.
As will be seen from the above equation (1), the value of C.sub.IS is greater due to the presence of C.sub.12 of about 3.sub.p F.